The integration of molecular and morphological approaches has produced substantial progress in understanding the higher classification of most major invertebrate groups. The striking exception to this is the Polychaeta. Neither the membership nor the higher classification of this group has been robustly established. Major inconsistencies exist between the only comprehensive cladistic analysis of Polychaeta using morphological data and the DNA sequence studies covering all or part of the taxon.

We have compiled a dataset of available nearly complete 18S ribosomal DNA sequences and collected an additional 22 sequences (20 Polychaeta in 19 taxa, one Myzostomida and one Phoronida) to obtain more comprehensive coverage of polychaete diversity for this gene. Analyses of the data do not resolve all inconsistencies among current hypotheses of polychaete phylogeny. They do support the recognition (in whole or part) of some clades such as the Eunicida, Phyllodocida and Terebellida that have been proposed on morphological grounds. Our analyses contradict the Canalipalpata and the Scolecida. Although the polychaete sister-group to the Clitellata is not clearly resolved in our analyses, the clitellates are always recovered as a derived clade within the Polychaeta. Increased taxon sampling is required to elucidate further the phylogeny of the Polychaeta.